Base width modulation or Early effect in Transistor
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6 سال پیش
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BASE WIDTH MODULATION OR EARLY
BASE WIDTH MODULATION OR EARLY EFFECT IN TRANSISTOR
In Active region emitter base junction is forward biased and collector base junction is reverse biased.
So the depletion layer width at emitter base junction is negligible as compared to depletion layer width at collector base junction.
The transition or space charge region is the region of uncovered charges on both sides of junction.
To maintain neutrality of charge, the number of uncovered charges on each side remains same.
As the voltage applied across the Collector base junction increases the transition region penetrates deeper into the base than that of collector.
Because the doping of base is relatively smaller than the collector, the penetration of depletion layer is more in base region.
Hence the collector depletion region is neglected and all the immobile charges are indicated in base region.
The modulation of effective base width by collector to base reverse bias voltage VCB is known as Early effect.
The junction voltage is given by
VB=𝑒𝑁𝐷𝑊2/2ε
ND = donar density
W=width of depletion layer
If reverse bias voltage increases, then the junction voltage VB increases which inturn increases width of depletion layer from above equation.
If width of depletion layer increases the effective base width WB reduces.
The decrease in base width WB with the increasing reverse bias has three consequences
There is less chance of recombination with in the base region. Hence, amplification factor alpha and transport factor beta increases with increase in magnitude of VCB
The concentration gradient of minority carriers is increased within the base and consequently the current due to minority carriers injected across the emitter junction increases with increase in VCB
Since the injected minority current across the emitter is proportional to gradient of minority carriers. Then emitter current increases with increase in VCB
For extremely large collector to base voltage the effective base width WB may be reduced to zero causing voltage breakdown of transistor. This phenomenon is called punch through.
In Active region emitter base junction is forward biased and collector base junction is reverse biased.
So the depletion layer width at emitter base junction is negligible as compared to depletion layer width at collector base junction.
The transition or space charge region is the region of uncovered charges on both sides of junction.
To maintain neutrality of charge, the number of uncovered charges on each side remains same.
As the voltage applied across the Collector base junction increases the transition region penetrates deeper into the base than that of collector.
Because the doping of base is relatively smaller than the collector, the penetration of depletion layer is more in base region.
Hence the collector depletion region is neglected and all the immobile charges are indicated in base region.
The modulation of effective base width by collector to base reverse bias voltage VCB is known as Early effect.
The junction voltage is given by
VB=𝑒𝑁𝐷𝑊2/2ε
ND = donar density
W=width of depletion layer
If reverse bias voltage increases, then the junction voltage VB increases which inturn increases width of depletion layer from above equation.
If width of depletion layer increases the effective base width WB reduces.
The decrease in base width WB with the increasing reverse bias has three consequences
There is less chance of recombination with in the base region. Hence, amplification factor alpha and transport factor beta increases with increase in magnitude of VCB
The concentration gradient of minority carriers is increased within the base and consequently the current due to minority carriers injected across the emitter junction increases with increase in VCB
Since the injected minority current across the emitter is proportional to gradient of minority carriers. Then emitter current increases with increase in VCB
For extremely large collector to base voltage the effective base width WB may be reduced to zero causing voltage breakdown of transistor. This phenomenon is called punch through.
6 سال پیش
در تاریخ 1397/06/18 منتشر شده
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